Method and device for estimating a signal to interference ratio (SIR) in wideband code division multiple access (WCDMA) systems

ABSTRACT

A method and device ( 100 ) for estimating a signal to interference ratio (SIR) of a signal transmitted from a first unit and to a remotely located second unit in a Wideband Code Division Multiple Access (WCDMA) wireless communication system. The transmitted TPC (Transmit Power Control) is checked and upon this result the SIR is determined. The checking of the TPC includes the estimation of the previous and the present power using a weighted contribution of the pilots and the data.

This patent application claims the benefit of priority from U.S.Provisional Patent Application Ser. No. 60/418,912 filed on Oct. 16,2002. This application incorporates by reference the entire disclosureof U.S. Provisional Patent Application Ser. No. 60/418,912.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to a method and device forestimating the signal-to-interference ratio (SIR) of signals transmittedin wireless communication networks through an air interface. Moreparticularly, the invention relates to a method and device for signal tointerference estimation in WCDMA (Wideband Code Division MultipleAccess) Radio Access Networks. Even more particularly, the inventionrelates to a robust and precise SIR estimation under conditions withnoise disturbed transmission channels and unknown contents of thetransmitted data.

BACKGROUND OF THE INVENTION

Power control is important for capacity and efficiency in mobile WCDMAsystems. For example, if a mobile transmitter/receiver unit, such as amobile radio terminal, is located close to a base stationtransmitter/receiver, the power level of signals, which are wirelesstransmitted over an air interface to the mobile unit from the basestation, in absence of adjustment, would be comparatively high. Thiscould interfere with transmissions to other mobile units located fartheraway from the base station. Conversely, the signal power oftransmissions from a mobile unit, which was located far from the basestation, could be comparatively weak. Accordingly, it has become commonpractice to provide a transmission power control in such wirelesscommunication systems.

Currently, power control is accomplished by estimating the signal tointerference ratio (SIR) of received signals. If the SIR of a signalreceived at a mobile unit is lower than a threshold value, a command oradjustment signal is sent to the transmitting base station to increasetransmission power. The command is sent on the reverse link of thecommunication system, which could either be the uplink or the downlink,depending on which link is controlled. If the estimated SIR is higherthan the threshold value, a command to decrease transmission power issent, or vice versa.

More specifically, as is well known to those skilled in the art, thesignal to interference ratio is a crucial quantity in the inner looppower control. In a typical power control system, see FIG. 1B, the outerloop sets the reference target SIR and the inner loop adjusts thetransmitted power such that the estimated SIR agrees with the referenceor target SIR. The inner loop controls the communication between a basestation and a mobile user equipment and vice versa. This is achieved bygiving power up or power down commands, that is, TPC (Transmit PowerControl, as mentioned above) equals one or zero respectively. To eachtransport block there is a cyclic redundancy check (CRC). If the decodedCRC is determined to be correct, the CRC error flag (CRCef) is set tozero, otherwise to one. Filtering or averaging the CRCef in anotherappropriate way obtains an estimate of the block error rate (BLER).Usually the outer loop consists of an appropriate regulator that adjuststhe reference SIR value depending on the discrepancy between thereference BLER and the estimated BLER. The purpose of the power controlis to keep the estimated BLER as close as possible to the referenceBLER.

The inner loop runs usually at such a frequency that a new SIR estimateis produced every slot. In the 3GPP specification it is suggested thatthe pilot symbols be used for the SIR estimation. The user equipment(UE), such as a mobile radio terminal, is required to produce a TPCcommand within 512 chips beginning from the first arrived propagationchannel path, see FIG. 2. The relative power difference between the datasymbols and the pilot symbols is known. The transmitted power change isalways done at the beginning of the pilot field. However, using only thepilots in estimating the SIR produces very noisy estimates, which inturn produces a large variance on the transmitted powers.

It is not desired to have such a variance of transmitted powers as boththe UE and the base-station benefit from a low variance in thetransmitted power. With a low variance of transmitted powers, e.g. alower average transmitted power can be achieved and thus additionalcapacity in the base-station and less power consumption at the UE isachieved.

U.S. Pat. No. 6,070,086 discloses the use of the data bits as well asthe pilots for estimating the SIR. The data bits are added coherently,whereby the sent bits have to be known. In U.S. Pat. No. 6,070,086 harddecisions are made on the data bits. This implies that few mistakes indecoding the sent data bits are made, if the channel quality is good.Thus they can consequently be useful in estimating a more precise SIR.However, the contrary is true if the channel is of poor quality due toe.g. noise or other disturbances.

Furthermore, in the above mentioned state-of-the-art method the powerestimates of the pilot and data symbols are given equal weights and thesent TPC command is always assumed to be received correctly. In practicethis is not always the case, thus resulting in inaccurate SIRestimation. It is believed that power is changed in one direction, i.e.up or down depending on the TPC data, but in reality it is changed inthe other direction, because the received TPC data has been modified bytransmission errors. This results in an inaccurate SIR estimation andcauses also high variance of the transmitted power, as mentioned above.

Therefore, a need exists for a robust and precise SIR estimate underconditions with noise disturbed transmission channels and unknowncontents of the transmitted data.

SUMMARY OF THE INVENTION

The present invention overcomes the above-identified deficiencies in theart by providing a robust and precise SIR estimate according to theappended independent claims.

The SIR estimation is performed for a signal transmitted from a firstunit and to a remotely located second unit in a Wideband Code DivisionMultiple Access (WCDMA) wireless communication system, whereby thesignal is transmitted through an air interface. A Transmit Power Control(TPC) verification is performed and the SIR estimation depends on theresult of the TPC verification.

According to an aspect of the invention, a method is provided,comprising the steps of verifying a transmitted Transmit Power Control(TPC) command, and giving a SIR estimation depending on the result ofsaid TPC verification.

According to another aspect of the invention, a device is provided,comprising a means for Transmit Power Control (TPC) verification (40)having an output signal, a means for SIR estimation (50), whereby theSIR estimation depends on said output of said TPC verification unit.

According to yet another aspect of the invention, a computer readablemedium is provided, having a plurality of computer-executableinstructions for performing the method according to the invention.

The SIR estimate according to the invention is operable under conditionswith noise disturbed transmission channels and unknown contents of thetransmitted data. The SIR estimation according to the invention is moreaccurate and robust than those of the prior art.

Averaging is performed only over one symbol and the need of making harddecisions on the symbols is eliminated, thus resulting in a simplerpower estimator. However, averaging can also be performed over more thanone symbol.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be described in thefollowing detailed disclosure, reference being made to the accompanyingdrawings, in which

FIG. 1A shows a block diagram illustrating a wireless communicationsystem employing embodiments of the invention,

FIG. 1B is an overview of the power control in WCDMA for a dedicatedphysical channel,

FIG. 2 shows a timing diagram of the transmit control power timing forWCDMA,

FIG. 3 illustrates the partitioning of slots for the downlink at the UE,

FIG. 4 illustrates the partitioning of slots for the up-link at UTRAN,whereby the control channel is the Dedicated Physical Control Channel(DPCCH) and the data channel is the Dedicated Physical Data Channel(DPDCH),

FIG. 5 is a flow chart illustrating an embodiment of the methodaccording to the invention, and

FIG. 6 shows an embodiment of a device according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

A more complete understanding of the method and device according to theinvention will be derived from the following detailed description ofembodiments of the invention in conjunction with the appended drawingsand claims. For this purpose, some definitions of terms will be given.

In FIG. 3 I₁ ^((n)) denotes the pilot symbols at time n; I₂ ^((n))denotes the data symbols in data field one and the TPC symbols; I₃^((n)) denotes the TFCI (Transport Format Combination Indicator) symbolsand the data symbols in data field two. As illustrated in FIG. 3, theTPC command is to be computed at the end of interval I₂ ^((n)). For aneasier understanding of the reasoning below, only the data symbols in I₂^((n)) and I₃ ^((n)) will be used for the downlink power estimation.

In FIG. 4, I₁ ^((n)) denotes the pilot symbols, TPC, TFCI, and FBI(Feedback Information) symbols, and I₂ ^((n)) denotes the data symbols.To simplify the analysis only the pilot symbols will be used in I₁^((n)).

Δ_(TPC) denotes the change of power in dB, which results from a TPCcommand, and Δ_(rel) is the relative power discrepancy between pilot anddata symbols in dB. Δ_(rel) is usually known. Otherwise Δ_(rel) is foundby filtering the quotient of the estimated power between pilot and datasymbols.

Generally, x is a complex number, and x* is the conjugate and |x| is theabsolute value. If x is a random variable, the expectation is denoted byE(x).

The despread received symbol is given by y_(j), wherebyy _(j) =h _(j) x _(j) +n _(j).

Here, the propagation channel is modelled by h_(j) for symbol j, thesent symbol is x_(j), and thermal noise, quantization noise, as well asthe intra/inter cellular interference noise is given by n_(j) for symbolj. The complex symbol x_(j) is scaled to have length one. The noise isassumed to be uncorrelated, zero mean, with variance σ². Only onemulti-path delay is considered here. The reasoning for severalmulti-path delays is done in the same manner, applying the belowreasoning for each delay. The reasoning below is also restricted to thedown-link since it is the slightly more complicated case and is modifiedfor the up-link case, if desired.

FIG. 5 shows an embodiment of the method according to the invention. InFIG. 5 the TPC verification and SIR estimation is performed as follows.P_(i) ^((n)) is the average received power for the symbol or a subset ofsymbols in interval I_(i) ^((n)). Denote the subset of symbols in I_(i)^((n)) as y_(jk) ^((n,i)), k=1, . . . , N_(i). Here, N_(i) is the numberof symbols used in interval I_(i) ^((n)), and j_(k) is enumerating theselected symbols, where k is the index for the subset. The averagereceived power is calculated in 5.10 as

$P_{i}^{(n)} = {\frac{1}{N_{i}}{\sum\limits_{k = 1}^{N_{i}}{y_{j_{k}}^{({n,i})}( y_{j_{k}}^{({n,i})} )}^{*}}}$

and the expectation of P_(i) ^((n)) is given by

${E\text{(}P_{i}^{(n)}\text{)}} = {{\frac{1}{N_{i}}{\sum\limits_{k = 1}^{N_{i}}{h_{j_{k}}}^{2}}} + \sigma^{2}}$

In this case it is assumed that we have no transmit diversity. Though,in the case of transmit diversity where the data symbols are encodedusing space-time transmit diversity (STTD) and two transmit antennas areused, the same formula is valid, which will be shown below.

The interference is estimated from the pilot symbols in 5.20 as

$N^{(n)} = {\frac{1}{N_{p} - 1}{\sum\limits_{j = 1}^{N_{p}}{{y_{j}^{({n,1})} - {\frac{1}{N_{p}}{\sum\limits_{k = 1}^{N_{p}}{y_{k}^{({n,1})}( x_{j}^{({n,1})} )}^{*}}}}}^{2}}}$Very often the interference is modelled as being a slow changingprocess. In this case it is possible to filter the variable N^((n)) toget a better estimate. Though, the remaining reasoning below holdswhether filtered or unfiltered noise estimates are used.

The SIR is estimated at time n as

$\begin{matrix}{{SIR}_{est}^{(n)} = {\frac{P^{(n)}}{N^{(n)}} - 1.}} & (1)\end{matrix}$

Here, P^((n)) is the estimated power, given byP ^((n)) =w ₃ P ₃ ^((n−1))·10^(0.1Δ) ^(TPC) ^((n−1)) +w ₁ P ₁^((n))·10^(0.1Δ) ^(rel) +w ₂ P ₂ ^((n))

depicted as 5.30.

One is subtracted in equation (1) to account for the noise. Herew_(i)≧0, for i=1, . . . , 3 and it isw ₁ +w ₂ +w ₃=1

Preferably, the weights are selected, for example, in relation to thecontributed energy. That is,

${w_{1} = \frac{N_{1}}{N_{1} + {10^{{- 0.1}\Delta_{rel}}N_{2}} + {10^{{- 0.1}\Delta_{rel}}N_{3}}}},{w_{2} = {\frac{10^{{- 0.1}\Delta_{rel}}N_{2}}{N_{1} + {10^{{- 0.1}\Delta_{rel}}N_{2}} + {10^{{- 0.1}\Delta_{rel}}N_{3}}}\mspace{14mu}{and}}}$$w_{3} = {\frac{10^{{- 0.1}\Delta_{rel}}N_{3}}{N_{1} + {10^{{- 0.1}\Delta_{rel}}N_{2}} + {10^{{- 0.1}\Delta_{rel}}N_{3}}}.}$

It is assumed that the TPC command at time n−1 has been receivedcorrectly at the base-station. The SIR estimator given by equation (1)is extended to use older information, i.e. information from slots beingtransmitted at times before n−1, if desired, by assuming that all TPCcommands are received correctly. However, this is limited because thepropagation channel changes due to fading. If knowledge about the rateof change of the propagation channel is available, a SIR estimator isused that controls the amount of old information depending on the rateof change of the propagation channel.

TPC is verified in 5.40 as follows.

It is Δ_(TPC) ^((n−1))=±Δ_(TPC) dB, depending on if we have a power up(+) or power down (−) command. Take{circumflex over (P)} _(est) ^((n)) =ŵ ₁ P ₁ ^((n))·10^(0.1Δ) ^(rel) +ŵ₂ P ₂ ^((n))  (2),

which only uses pilot and data symbols after the transmitted powerchange. Then, it isP _(est) ^((n−1)) = w ₁ P ₁ ^((n−1))·10^(0.1Δ) ^(rel) + w ₂ P ₂^((n−1)) + w ₃ P ₃ ^((n−1))  (3),

which is the power in the previous slot counted from where the pilotbegins.

The weights in equation (2) and (3) obey the same conditions as theweights in equation (1), as mentioned above.

If a power up command is sent to the base station, the command has beencorrectly received, if| P _(est) ^((n−1))·10^(0.1Δ) ^(TPC) −{circumflex over (P)} _(est)^((n)) |<c| P _(est) ^((n−1))·10^(−01Δ) ^(TPC) −{circumflex over (P)}_(est) ^((n))|  (4)

Here, c is a constant. If c=1 it means that the difference in distancefrom the previous estimated power at time n−1 is compared, P _(est)^((n−1))·10^(±0.1Δ) ^(TPC) , given a power up or down command, to theestimated power in slot n, {circumflex over (P)}_(est) ^((n)). If c isset to c>1, then a bias towards picking the sent TPC command isintroduced. This is used when the power estimates are corrupted by a lotof noise.

In 5.50, SIR is estimated as follows, depending on the result of the TPCverification from 5.40. If equation (4) is true, i.e. the power commandhas been received correctly, the estimated SIR at time n is according toone embodiment of the invention:

$\begin{matrix}{{SIR}_{est}^{(n)} = {\frac{{w_{3}{P_{3}^{({n - 1})} \cdot 10^{0.1\Delta_{TPC}}}} + {w_{1}{P_{1}^{(n)} \cdot 10^{0.1\Delta_{rel}}}} + {w_{2}P_{2}^{(n)}}}{N^{(n)}} - 1}} & (5)\end{matrix}$on the other hand, if equation (4) is false, i.e. the power command hasnot been received correctly, the estimated SIR at time n is according toanother embodiment of the invention:

$\begin{matrix}{{SIR}_{est}^{(n)} = {\frac{{w_{3}{P_{3}^{({n - 1})} \cdot 10^{{- 0.1}\Delta_{TPC}}}} + {w_{1}{P_{1}^{(n)} \cdot 10^{0.1\Delta_{rel}}}} + {w_{2}P_{2}^{(n)}}}{N^{(n)}} - 1}} & (6)\end{matrix}$

Similarly, in another embodiment of the invention, if the sent commandis a down command it is checked, if| P _(est) ^((n−1))·10^(−0.1Δ) ^(TPC) −{circumflex over (P)} _(est)^((n)) |<c| P ^((n−1))·10^(01Δ) ^(TPC) −{circumflex over (P)} _(est)^((n))|is fulfilled.

This concludes the TPC verification and the SIR is estimated usingequation (5) or (6). It is possible to include more prior informationinto P _(est) ^((n−1)) in equation (4) if so needed. However, asmentioned above, caution should be taken if the propagation channelchanges rapidly.

In a further embodiment of the invention, the data symbols are STTD(space-time transmit diversity) encoded. In this case the power iscomputed by taking the absolute value of the symbols. However, thesymbols must be summed-up pair-wise as will become clear from thefollowing reasoning.

When the data symbols are STTD encoded, two consecutive received symbolsare described byy _(j) =h ⁽¹⁾ x _(j) −h ⁽²⁾ x* _(j+1), andy _(j+1) =h ⁽¹⁾ x _(j+1) +h ⁽²⁾ x* _(j).Where h⁽¹⁾ and h⁽²⁾ denotes the true propagation channel estimates forantenna one and two.It is|y _(j)| ² =|h ⁽¹⁾| ² +|h ⁽²⁾| ² −(h ⁽¹⁾)*h ⁽²⁾ x* _(j) x* ^(j+1) −h⁽¹⁾(h ⁽²⁾)*x _(j) x _(j+1), and|y _(j+1)| ² =|h ⁽¹⁾| ² +|h ⁽²⁾| ² +(h ⁽¹⁾)*h ⁽²⁾ x* _(j) x* _(j+1) +h⁽¹⁾(h ⁽²⁾)*x _(j) x _(j+1).Adding the squared symbols results in|y _(j)| ² +|y _(j+1)| ² =2(|h ⁽¹⁾| ² +|h ⁽²⁾| ² ),which coincides with the case of no transmit diversity.

FIG. 6 shows an embodiment of a device 100 of the invention. Device 100comprises means 10 for calculating the averaged received power, means 20for estimating the interference, means 30 for estimating the power,means 40 for TPC verification and means 50 for SIR estimation dependingon the output of means 40.

In a further embodiment of the invention, respective signals 3 aretransmitted from a base station 2 and received at a mobile unit 1.

In yet another embodiment of the invention, respective signals 3 aretransmitted from a mobile unit 1 and received at a base station 2.

It is thus to be emphasised that the principle of the invention isapplicable to both the downlink and the uplink case.

Furthermore, it should be emphasised that the term“comprises/comprising” when used in this specification is taken tospecify the presence of stated features, integers, steps or componentsbut does not preclude the presence or addition of one or more otherfeatures, integers, steps, components or groups thereof.

The present invention has been described above with reference tospecific embodiments. However, other embodiments than the preferredabove are equally possible within the scope of the appended claims.

1. A method for use in a Wideband Code Division Multiple Access (WCDMA)wireless communication system for estimating a signal to interferenceratio (SIR) of a signal transmitted from a first unit to a remotelylocated second unit in said WCDMA wireless communication system, saidsignal being transmitted through an air interface and comprising pilotand data symbols, the method comprising: verifying, by the first unit, atransmitted Transmit Power Control (TPC) command, by: determining, bythe first unit, when said TPC command has been correctly received, andweighting, by the first unit, said pilot and data symbols wherein saidweighting comprises taking into account a power change in said datasymbols due to a prior TPC change; and giving, by the first unit, a SIRestimation depending on the result of said verifying the transmitted TPCcommand.
 2. The method according to claim 1, comprising encoding saiddata symbols using space-time transit diversity (STTD).
 3. The methodaccording to claim 1, wherein interference is estimated from said pilotsymbols.
 4. The method according to claim 3, wherein the estimatedinterference is filtered.
 5. The method according to claim 1, whereinthe first unit is a base station and the second unit is a mobile unit.6. The method according to claim 1, wherein the first unit is a mobileunit and the second unit is a base station.
 7. A computer readablemedium having a plurality of computer-executable instructions forperforming the method according to claim 1, comprising: a program modulefor TPC verification giving instructions to a computer, and a programmodule for SIR estimation giving instructions to a computer, dependingon the Transmit Power Control (TPC) verification.
 8. The methodaccording to claim 1, wherein said giving a SIR estimation depending onthe result of said TPC verification comprises: when said TPC command hasbeen correctly received, the estimated SIR at time n is given as${SIR}_{est}^{(n)} = {\frac{{w_{3}{P_{3}^{({n - 1})} \cdot 10^{0.1\Delta_{TPC}}}} + {w_{1}{P_{1}^{(n)} \cdot 10^{0.1\Delta_{rel}}}} + {w_{2}P_{2}^{(n)}}}{N^{(n)}} - 1}$ and when said TPC command has not been correctly received, theestimated SIR at time n is given as${{{{SIR}_{est}^{(n)} = {\frac{{w_{3}{P_{3}^{({n - 1})} \cdot 10^{{- 0.1}\Delta_{TPC}}}} + {w_{1}{P_{1}^{(n)} \cdot 10^{0.1\Delta_{rel}}}} + {w_{2}P_{2}^{(n)}}}{N^{(n)}} - 1}};{{{where}\mspace{14mu} w_{i}} \geq 0}},{{{for}\mspace{14mu} i} = 1},\ldots\mspace{14mu},3,P_{l}^{(n)}}\mspace{14mu}$is the average received power for the symbol or a subset of symbols ininterval I_(i) ^((n)), N^((n)) is the estimated interference at time n,Δ_(TPC) is a change of power in dB, resulting from a prior TPC command,and Δ_(rel) is a relative power discrepancy between pilot and datasymbols in dB.
 9. A device for estimating a signal to interference ratio(SIR) of a signal transmitted from a first unit and to a remotelylocated second unit in a Wideband Code Division Multiple Access (WCDMA)wireless communication system, said signal being transmitted through anair interface and comprising pilot and data symbols, wherein said devicecomprises; a Transmit Power Control (TPC) verification means included inthe first unit having an output signal, wherein said TPC verificationmeans weighs said pilot and data symbols by taking into account a powerchange in said data symbols due to a prior TPC change and determineswhen a TPC command have been correctly received; and a means for SIRestimation, using said output signal as input signal and being arrangedto estimate the SIR depending on said output signal of said TPCverification means.
 10. The device according to claim 9, wherein saiddata symbols are encoded using space-time transmit diversity (STTD). 11.The device according to claim 9, further comprising a means forestimating interference from said pilot symbols.
 12. The deviceaccording to claim 11, further comprising a filter for filtering saidestimated interference.
 13. The device according to claim 9, wherein thefirst unit is a base station and the second unit is a mobile unit. 14.The device according to claim 9, wherein the first unit is a mobile unitand the second unit is a base station.